1. Field of the Invention
The present invention concerns a gas laser device that emits ultraviolet rays, especially a gas laser device that effects laser operation at long laser pulse width in an ArF excimer laser device and a fluoride laser device.
2. Description of Related Art
Higher resolution is demanded for projection exposure devices for the production of semiconductor integrated circuits as they become smaller and more integrated. Thus, the wavelength of exposure light emitted from an exposure light source becomes shorter, and gas laser devices that emit ultraviolet rays such as ArF excimer laser devices and fluoride laser devices would be effective as the next generation of light sources for semiconductor exposure.
A laser gas comprising a gas mixture of fluorine (F2) gas, argon (Ar) gas and a rare gas, such as neon (Ne), as buffer gas in an ArF excimer laser device or a gas mixture of fluorine (F2) and a rare gas, such as helium (He), as buffer gas in a fluorine laser device serving as the laser medium is excited by discharge within a laser chamber in which the laser gas is enclosed at several hundred kPa.
Since the spectral width of laser light that is emitted in an ArF excimer laser device is broad, at 400 pm, the spectral width must be set at a narrow band range below 1 pm to avoid the problem of color aberration in projection optical systems of exposure devices. The spectral width can be narrowed by disposing a band-restricting optical system comprising an expansion prism and a diffraction grating in a laser resonator.
Incidentally, the core oscillation wavelength in an ArF excimer laser device is 193.3 nm, which is shorter than the 248 nm core oscillation wavelength of a KrF excimer laser device currently used as an exposure light source. Consequently, the damage inflicted on quartz, the glass used in projection lens systems of exposure devices, such as steppers, is greater than that when using a KrF excimer laser device, which shortens the life of the lens system.
Quartz damage may be the formation of color centers, due to two photon absorption, and compaction (elevation of refractive index). The former appears as a decrease of the transmittance while the latter appears as a decrease of the resolution of the lens system. This effect is inversely proportional to the laser pulse width (Ti) defined by the following expression when the laser pulse energy is constant
Tis=(∫T(t)dt)2/∫(T(t))2dtxe2x80x83xe2x80x83(1) 
where T(t) represents the periodic laser shape.
This definition of the laser pulse width Tis is explained here. Assuming that damage to optical devices arises due to the absorption of two photons, the damage D that accumulates per pulse would be given by the following expression since damage is proportional to the square of the laser photointensity:
D=kxc2x7∫(P(t))2dtxe2x80x83xe2x80x83(2) 
where k represents a constant determined by the substance while P(t) is the periodic laser strength (MW).
Laser strength P(t) can be separated into time and energy via the following expression:
P(t)=Ixc2x7T(t)/∫(T(txe2x80x2))dtxe2x80x2xe2x80x83xe2x80x83(3) 
where, I represents energy (mJ) and T(t) represents the periodic laser shape.
I develops when P(t) is periodically integrated, and I would be 5 mJ in the case of an ArF excimer laser device.
Here, damage D would be represented by the following expression when expression (3) is substituted for expression (2):
D=kxc2x7I2xc2x7∫(T(t)/∫T(txe2x80x2)dtxe2x80x2)2dt=kxc2x7I2xc2x7∫(T(t)2dt/(∫T(t)dt)2 
When expression (1) is substituted, the result would be as follows:.
D=kxc2x7I2/Tisxe2x80x83xe2x80x83(5) 
Pulse width Tis, which is inversely proportional to damage D, would be defined by expression (1) since kxc2x7I2 (I is held constant) is a constant according to expression (5).
The laser pulse width has been defined in the past by the full-width half maximum (FWHM) of the periodic laser shape. When the laser pulse width is defined by the full-width half maximum, the value would remain the same even if the periodic laser shapes were to mutually differ, as shown by the model in FIG. 8. However, the continuous duration of the actual laser pulse is longer if it is triangular than if it is rectangular in the example shown in FIG. 8. In addition, the triangular shape is longer than the rectangular shape shown in FIG. 8 of the laser pulse width Tis defined in expression (1). For example, in the case shown in FIG. 8, the triangular laser pulse width Tis is double the rectangular laser pulse width Tis.
Extension of the laser pulse width Tis (pulse stretch) is desirable since the decrease in the transmittance due to the absorption of two photons and the decrease in the resolution due to compaction are inversely proportional to the laser pulse width Tis given by expression (1) when the laser pulse energy is constant.
The repetition rate of oscillation operation (hereinafter termed repetition rate) in a commercial, narrowed band range ArF excimer laser device for exposure is 1 kHz and the laser light output is commonly 5 W. Laser pulse width Tis should exceed 30 ns to avoid damage to the optical system mounted on a semiconductor exposure device.
As indicated above, pulse stretch that extends laser pulse width Tis is sought to reduce damage to the optical system mounted on an exposure device, but this pulse stretch is requested in light of the following points as well.
The resolution R of an image of a mask having a circuit pattern that is projected via a projection lens on a workpiece, such as a wafer having a photoresist applied to it, and the depth of focus DOF in a projection exposure device are represented by the following expressions:
R=k1xc2x7xcex/NAxe2x80x83xe2x80x83(6) 
DOF=k2xc2x7xcex/(NA)2xe2x80x83xe2x80x83(7)
Here, k1 and k2 represent the coefficients that reflect the resist characteristics, xcex represents the wavelength of exposure light emitted from the exposure light source and NA represents the numerical apertures.
To enhance resolution R, the wavelength of exposure light is shortened and the number of apertures is raised, as is clear from expression (6); but, the depth of focus DOF is diminished to the extent that these are implemented, as indicated in expression (7). The spectral line width of exposure light must be made narrower since the effects of color aberration are increased as a result. Specifically, still narrower spectral line width of laser light emitted from an ArF excimer laser device is sought.
The fact that the spectral line width of laser light becomes narrower as the laser pulse width is stretched was stated in Proc. SPIE Vol. 3679 (1999) 1030-1037, and experiments by the inventors have corroborated this point. Specifically, further narrowing of the spectral line width of laser light is sought to enhance resolution R, and pulse stretch of the laser pulse width is essential for that.
Laser pulse width Tis must be elongated to enhance the resolution and that avoids damage to the optical system of an exposure device, as indicated above. Laser pulse width Tis is known to be dependent on the concentration of fluorine gas in the laser gas enclosed in the laser chamber (source: Proc. SPIE Vol. 3679 (1999), 1030-1037), and laser pulse width Tis can be stretched so that Tisxe2x89xa730 ns by adjusting the concentration of fluorine gas.
A method of forming a laser pulse such that Tisxe2x89xa730 ns was proposed by the inventors in Patent Application No. Hei-11-261628 by carrying out a laser oscillation operation by the initial half-cycle of the discharge oscillation current waveform of a pulse that reverses the polarity and by at least one subsequent half cycle.
Higher resolution, higher throughput, lower damage to quartz are required of ArF excimer laser devices and of fluoride laser devices that are viable candidates for the next generation of semiconductor exposure light sources. However, pulse stretch that has the effects of raising the resolution and lowering the damage, and raising the repetition rate to raise the throughput are incompatible techniques from the perspective of maintaining the stability of discharge characteristics, and compatibility is difficult to realize.
The present invention was devised in light of the aforementioned problems associated with conventional techniques. The purpose is to provide an ArF excimer laser device and a fluoride laser device for exposure that permits pulse stretch even if the repetition rate should exceed 2 kHz.
To attain aforementioned purpose, an ArF excimer laser device in accordance with the present invention is provided with a pair of laser discharge electrodes disposed in a laser chamber connected to the output terminal of a magnetic pulse compression circuit and a peaking capacitor that is connected in parallel with the pair of laser discharge electrodes, so that primary current that infuses energy from the magnetic pulse compression circuit to the discharge electrodes via the peaking capacitor overlaps secondary current that infuses energy from the capacitor in the final stage of the magnetic pulse compression circuit for charging the peaking capacitor to the discharge electrodes, the oscillation cycle of the secondary current being set longer than the oscillation cycle of aforementioned primary current, and a pulse of laser oscillation operation being effected by the initial half-cycle of the discharge oscillation current waveform that reverses the polarity of aforementioned primary current overlapped by the secondary current and by at least two half-cycles continuing thereafter.
In this case, the magnetic pulse compression circuit is provided with a magnetic pulse compression unit comprising a semiconductor switch, a capacitor having more than one stage and a magnetic switch, the inductance of a circuit loop formed from the peaking capacitor and the main discharge electrodes being 5 to 8 nH, the total gas pressure within the laser chamber being 2.5 to 3.7 atmospheres, the fluorine partial pressure being under 0.1%, the rise time until breakdown of the voltage applied to aforementioned main discharge electrodes develops being 30 to 80 ns, and the relation between the capacitance Cp of the peaking capacitor and the capacitance Cn of the capacitor for charging the peaking capacitor in the final stage of the magnetic pulse compression circuit is 0.45 less than Cp/Cn less than 0.75. In this case, the capacitance Cp of the peaking capacitor should be under 10 nF.
In addition, a preionization electrode is disposed near one of the main discharge electrodes, and the capacitance Cc of a preionization capacitor connected in series to the preionization electrode and connected in parallel with the peaking capacitor is 5% or less of the capacitance Cp of the peaking capacitor.
Furthermore, the reflectance of the output mirror of the optical resonator disposed within the laser chamber exceeds 50%.
In addition, the number of round trips of the optical resonator exceeds six.
Still further, the length of the main discharge electrodes is 550 to 750 mm, and the separation between electrodes should be 14 to 18 mm.
The fluoride laser device pursuant to the present invention is provided with a pair of laser discharge electrodes disposed in a laser chamber connected to the output terminal of a magnetic pulse compression circuit and a peaking capacitor that is connected in parallel with aforementioned pair of laser discharge electrodes, and is structured so that primary current that infuses energy from the magnetic pulse compression circuit to the discharge electrodes via the peaking capacitor overlaps secondary current that infuses energy from the capacitor in the final stage of the magnetic pulse compression circuit for charging the peaking capacitor to the discharge electrodes, the oscillation cycle of the secondary current being set longer than the oscillation cycle of the primary current, and a pulse of laser oscillation operation being effected by the initial half-cycle of the discharge oscillation current waveform that reverses the polarity of the primary current overlapped by the secondary current and by at least two half-cycles continuing thereafter.
The present invention is structured so that primary current that infuses energy from the magnetic pulse compression circuit to the discharge electrodes via a peaking capacitor overlaps secondary current that infuses energy from the capacitor for charging the final stage of the peaking capacitor in the magnetic pulse compression circuit to the discharge electrodes, the oscillation cycle of the secondary current being set longer than the oscillation cycle of the primary current, one pulse of laser oscillation operation being effected by the initial half-cycle of the discharge oscillation current waveform that reverses the polarity of the primary current overlapped by the secondary current and by at least two half-cycles continuing thereafter, results in an ArF excimer laser device and a fluoride laser device of high repetition rate, pulse stretch, line-narrowed when the repetition rate exceeds 2 kHz.
The principles and embodiments of the present invention are explained below in greater detail with reference to the accompanying drawings.